NATE HVAC flashcards
NATE HVAC · Build recall with a focused review session.
32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Superheat definition
- The difference between the measured suction-line temperature and the saturated evaporating temperature at the same pressure, showing how far the vapor has risen above saturation.
- One ton of refrigeration
- A cooling rate equal to 12,000 Btu per hour or 200 Btu per minute, the heat removal needed to freeze one ton of ice in 24 hours.
- Ohm's law
- Voltage equals current times resistance, written E = I × R. For example, 240 volts across 24 ohms produces 10 amps of current flow.
- Nitrogen purge while brazing
- A slow flow of dry nitrogen through the tubing during brazing displaces oxygen so carbon and oxide scale cannot form inside the refrigerant circuit.
- Target superheat fixed orifice
- With a fixed orifice or piston, charge is correct when measured superheat matches the manufacturer chart, commonly 10 to 15°F at design indoor wet bulb and outdoor temperature.
- Subcooling TXV target
- On a thermostatic expansion valve system, liquid-line subcooling of roughly 8 to 12°F indicates a full liquid column and correct refrigerant charge.
- Sensible heat equation
- Sensible Btu per hour equals 1.08 times standard airflow in cubic feet per minute times the dry-bulb temperature difference across the coil.
- R-410A characteristics
- A near-azeotropic blend running roughly 50 to 60 percent higher pressures than R-22, using POE oil, and requiring gauges and hoses rated for its pressure.
- Velocity pressure formula
- Air velocity in feet per minute equals 4005 times the square root of velocity pressure measured in inches of water column with a pitot tube.
- Typical branch duct velocity
- Supply branch ducts are usually sized for 400 to 700 feet per minute, while trunk lines run 600 to 900 feet per minute to limit noise and friction.
- External static pressure
- The sum of supply and return duct pressure drops measured at the air handler, typically limited to about 0.5 inch water column in residential systems.
- Clocking the gas meter
- Input Btu per hour equals 3600 divided by the seconds for one cubic foot of meter rotation, multiplied by the gas heating value in Btu per cubic foot.
- Natural gas manifold pressure
- Most natural gas appliances are set near 3.5 inches water column manifold pressure, with about 5 to 7 inches inlet supply pressure while firing.
- Temperature rise check
- Temperature rise in degrees Fahrenheit equals furnace output Btu per hour divided by 1.08 times blower cfm, and must fall within the nameplate rise range.
- Balance point
- The outdoor temperature at which heat pump heating capacity exactly equals building heat loss; below that point supplemental or auxiliary heat must make up the difference.
- Defrost cycle
- A timed or demand-initiated reverse-cycle operation that melts frost from the outdoor coil, typically terminating on coil temperature or a time limit.
- Emergency heat mode
- A control setting that locks out the compressor and uses only electric resistance or other backup heat, often selected manually after a compressor failure.
- Reversing valve operation
- On most residential heat pumps the solenoid is energized in heating, routing discharge gas to the indoor coil; it is de-energized for cooling mode.
- Nozzle flow versus pressure
- Oil nozzle flow varies with the square root of pressure, so a nozzle rated 1.00 gph at 100 psi delivers less than rated flow at a lower pump pressure.
- Cad cell resistance
- A cadmium sulfide flame sensor reads roughly 400 to 1000 ohms when flame is present; the primary control locks out when resistance climbs above about 1600 ohms.
- Number two fuel oil heat content
- A gallon of number two heating oil contains about 140,000 Btu, so a 1.00 gph nozzle firing at 100 psi releases roughly 140,000 Btu per hour.
- Hydronic gpm formula
- Gallons per minute equals Btu per hour divided by 500 times the temperature drop, because one gallon per minute carries 500 Btu per hour per degree.
- Boiler relief valve setting
- Residential hot water boilers use a pressure relief valve typically set at 30 psi, protecting the boiler and piping from overpressurization and unsafe conditions.
- Diaphragm expansion tank precharge
- The tank air charge is set to the system cold fill pressure, commonly 12 to 15 psi, and checked before the tank is connected to the loop.
- Three-phase power formula
- Three-phase watts equal volts times amps times 1.732 times power factor, used to estimate motor input power from measured current and voltage.
- Economizer changeover
- Rooftop economizers admit outdoor air for free cooling when outdoor enthalpy or dry-bulb temperature falls below the return air condition.
- TEV superheat setting
- Commercial evaporators using a thermostatic expansion valve are typically set for 4 to 6°F superheat at the coil outlet to fill the coil without floodback.
- Refrigerant safety group A2L
- A classification for mildly flammable, low-toxicity refrigerants such as R-32 and R-454B, carrying charge limits and ignition-source precautions in the mechanical code.
- Suction riser velocity
- Vertical suction risers must maintain at least 700 to 1500 feet per minute so entrained oil can be carried back up to the compressor.
- Reach-in cooler temperature
- Product space in a reach-in cooler is held near 35 to 38°F, with a coil temperature roughly 10 to 12°F colder than the box.
- Defrost termination temperature
- Electric defrost on a walk-in freezer terminates when the coil sensor reaches about 45 to 55°F, after which fans restart following a delay.
- Capillary tube critical charge
- Capillary tube systems have no receiver and are charged by weight; adding refrigerant to correct a partial frost pattern usually overcharges the system.